The distillation separation of streams such as air and natural gas streams which are contaminated with lighter impurities is well known. Typically, an overhead containing the volatile component contaminated with lighter impurities and a bottom fraction comprising the heavy component is obtained. Technology advancements in many industrial fields have required products of higher purity than are normally obtained in the conventional distillation processes. The attainment of higher purity products has required further processing and distillation to effect removal of the lighter impurities from the volatile component. Due to these additional process steps substantial amounts of energy or additional equipment or both to achieve a higher level of purity of the volatile component is required. Ultra high purity product for example is required in the area of semiconductors and integrated circuits. Even though such technologies require ultra high purity product, sometimes the volumes required are not sufficient to support a plant dedicated to ultra high purity operation. Other technologies applications may not require the elaborate processing necessary to produce the ultra high purity product. Those technologies cannot afford the cost factor associated with the processing of stream required to produce ultra high purity components. Accordingly, there is substantial interest in the distillation field, and particularly in the cryogenic field, to produce a component of one purity and the same component in ultra high purity. In that way, larger units can be built and the products distributed to the respective technologies. Representative art showing the distillation of streams and the separation of components therein into fractions are as follows:
U.S. Pat. No. 4,662,917 discloses a single column process for the production of nitrogen and oxygen. In that process, air is freed of its impurities and cooled to its dew point temperature and introduced into a single column for separation into its components. An oxygen-rich heavy stream is removed from the bottom of the column and removed as a product. A nitrogen-rich fraction is removed from the top of the column. A portion is condensed against oxygen-rich heavy stream in a boiler/condenser wherein a portion is returned as reflux to the single column and a portion removed as product.
U.S. Pat. No. 4,871,382 discloses a conventional dual column process for the separation of air into its components which includes a side arm column for the recovery of argon. In that process, air is introduced to a high pressure column wherein a nitrogen-rich fraction is generated at the top of the column and an oxygen-rich fraction is generated at the bottom of the column. A portion of the oxygen-rich fraction and nitrogen-rich overhead fraction is introduced to a low pressure column wherein further fractionation is effected. A nitrogen-rich overhead is recovered as product from the low pressure column and oxygen, either in liquid or gaseous form, is recovered from the bottom of the low pressure column. An argon stream is obtained by removing an argon-rich side stream from the low pressure column and fractionating in a side arm column wherein argon is removed as an overhead fraction and oxygen is removed as a bottom fraction.
U.S. Pat. No. 4,902,321 discloses a process for the production of ultra high purity nitrogen by the cryogenic distillation of an air stream. As in the above references, an oxygen-rich stream is generated at the bottom of a distillation column, and a nitrogen-rich stream is generated as an overhead in the column.
In contrast to the processes described in the aforementioned patents, a portion of the nitrogen overhead is condensed in a boiler/condenser wherein the impurities in the overhead are removed as a lighter impurity. The condensate from this boiler/condenser is then let down in pressure and vaporized in the same boiler/condenser to provide a high purity nitrogen product stream which has lower concentration of lighter impurities. Clearly, the disadvantage of this system is that the high purity nitrogen product is at a pressure which is lower than the distillation column pressure.
U.S. Pat. No. 5,049,173 discloses a process for producing ultra high purity oxygen from an air stream. In the process, as with other cryogenic distillation processes for the separation of air, an oxygen-rich bottoms fraction and a nitrogen-rich overhead fraction is produced from the distillation column. An oxygen-containing stream essentially free of heavy components is charged to a second fractionation column wherein the oxygen is stripped of volatile impurities and an ultra high purity liquid oxygen fraction is obtained.